Fraunhofer Institute for Wood Research Researchers at the Fraunhofer WKI develop humin-based binders for wood-based materials
In collaboration with industrial partners, scientists at the Fraunhofer WKI have developed concepts for the material utilization of humins. Humins are dark, highly viscous compounds. They accrue as a by-product during the production of polyethylene furanoate (PEF), a new bio-based substitute for the bulk plastic polyethylene terephthalate (PET). As a result of their complex chemical structure, humins are suitable for a wide range of applications, for example as binders. According to calculations, tens of thousands of tons of humins will accrue every year in the medium to long term, for which no applications currently exist.
In a research project conducted in collaboration with the companies Avantium, Delignit and Pfleiderer, the project partners were able to successfully utilize humins as binders for wood-based materials. The humins, which accrue as a by-product, are heterogeneous, polydisperse macromolecules with a furan structure and alcohol, ketone and aldehyde groups. They contain low-molecular and high-molecular fractions. When heat is applied, they cure duroplastically.
The aim was not to chemically modify humins but instead to formulate them with suitable hardeners, cross-linkers and additives. The researchers produced plywood and single-layer particle boards in the Fraunhofer WKI technical center and examined the achievable board properties. The low-molecular-weight fraction proved to be particularly suitable. Furthermore, the researchers succeeded in conducting the application using conventional facilities. The pressing times were moderate. The equipment could be cleaned simply with water. In addition, the project partners discovered that humins can be readily mixed with standard resins. Humin-bound wood-based materials can be processed using customary methods, such as sawing, drilling or sanding. A number of particle-board variants achieved the transverse tensile strength required for type P2; the thickness swelling is, however, significantly higher than for conventionally bound materials. Humin-bonded plywood was produced with up to seven layers. Without pre-treatment, the quality of the bonding achieved a tensile shear strength of up to 3 N/mm². Pre-treatment in accordance with EN 314-2 – i.e. in the form of 24-hour cold-water immersion – has not yet been withstood by humin-bonded plywood.
Furthermore, the partners discovered that humins are suitable for impregnating and modifying wood: they improve the durability against wood-destroying fungi by one-to-two durability classes. In addition, they reduce the swelling and shrinkage behavior of wood and have a hydrophobic effect. Moreover, cured humins fulfill the requirements of the ecological-toxicity test in accordance with OECD guideline 208 (sec. 8, EN 13432).
Humins are a promising component for increasing the proportion of renewable raw materials in binders for wood-based materials - the research project is therefore providing a contribution towards the achievement of the German government’s “Mission Klimaneutralität 2045” (Mission: Climate Neutrality 2045) climate targets and the bioeconomy. Assuming it is possible to further improve the performance capabilities of the binders, for example in terms of strength and water stability, a widely available adhesive alternative would be the result. This would constitute a building block on the path towards a resource-efficient economy based on renewable rather than fossil raw materials.
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